Ink jet recording is a process of ejecting an ink composition as small droplets through minute nozzles to record letters and images (hereinafter also simply referred to as “images”) on a surface of a recording medium. The ink jet recording processes that have come in practice include a process of converting an electric signal to a mechanical signal using an electrostrictive element, thereby intermittently ejecting an ink composition stored in a nozzle head portion to record letters and images on a surface of a recording medium; a process of rapidly heating an ink composition stored in a nozzle head portion at a part very close to an ejection portion to generate bubbles, and intermittently ejecting the ink composition by volume expansion due to the bubbles to record letters and images on a surface of a recording medium; and the like.
Further, as an ink composition for ink jet recording, there is commonly used one in which various dyes are dissolved in water, an organic solvent or a mixed solution thereof, from the aspects of safety and print characteristics. In various characteristics, more strict conditions are required, compared to an ink composition for writing things such as a fountain pen and a ball pen.
In particular, in recent years, ink jet printers have come to be employed for the preparation of printed matter for advertisement. Printed matter prepared using an ink composition as described above is placed not only indoors as a matter of course, but also outdoors in some cases. Accordingly, the printed matter comes to be exposed to various lights including sunlight and the outdoor air (ozone, a nitrogen oxide, a sulfur oxide or the like), so that the phenomenon that image quality is deteriorated has occurred. In particular, the deterioration in a black colored portion is outstanding.
Consequently, black ink compositions excellent in light resistance and gas resistance have been developed. Together with these characteristics, it is also necessary to satisfy performances generally required for an ink composition for ink jet recording, such as no occurrence of nozzle clogging and no changes in composition and physical properties during storage, so long as it is the ink composition used for ink jet recording. The characteristics such as light resistance and gas resistance are largely dependent on a colorant used in the ink composition. When even a colorant excellent in light resistance and the like is decomposed during storage to generate air bubbles in the ink or largely changes the pH of the ink, it is necessary to develop usage preventing these problems to use. This is because the air bubbles in the ink block an ink flow path, so that they contribute to print omission cause due to pressure loss, and because a large decrease in pH during storage in an ink cartridge causes the corrosion of a nozzle plate. The corrosion of the nozzle plate deteriorates water repellency, so that it contributes to flight deflection of ink droplets to bring about deterioration in image quality.
As one of the countermeasures against such problems, pH adjustment in the preparation of the ink is conceivable.
As ink pH adjusting techniques proposed, there are a technique of adjusting the pH with an organic amine salt of a dicarboxylic acid for clogging prevention and recovery, high image quality, safety to the human body, and the like (see patent documents 1 to 3), a technique of adjusting the pH with an amine and an oxy acid for prevention of drying and improvements in print quality and dissolution stability (see patent document 4), and a technique of adjusting the pH with a glycolic acid (see patent document 5) However, these are not pH adjusting techniques for solving the above-mentioned problems.
Further, as a pH adjusting technique for keeping a change in ink pH with time to a minimum, there is a technique of adjusting the pH with an amine containing a substituent group selected from a sulfonic acid group, a carboxylic acid group and a halogen atom (see patent document 6). However, the decomposition of a colorant is not suppressed in this technique. In order to solve the above-mentioned problems, it becomes necessary to adjust the pH to a value at which the colorant is less decomposed, with a compound which accelerates the decomposition of the colorant as little as possible, in addition to making the pH difficult to change with time.
Patent Document 1: JP 4-339874 A
Patent Document 2: JP 4-339875 A
Patent Document 3; JP 4-339876 A
Patent Document 4: JP 5-148437 A
Patent Document 5: JP 5-339527 A
Patent Document 6: JP 6-166840 A